Tranny leak while towing

In the past 2 weeks I've found the tranny is leaking while flat towing the jeep. The tranny is in P and the tcase is in N. I've only noticed it recently. I know the o-ring in the dipstick tube needs to be replaced. Could that be my problem or possibly something else? It doesn't leak while driving. The only changes I've made in this time is new gears f/r and new radiator. It was leaking before I changed the radiator though.
Suggestions? Thanks.
tcase is NP229, tranny is TF904

The leak is at the dipstick/fill tube.

I'm making some assumptions here that may not apply to the 904 or even the 727 4X tranny. But on the 727 2X tailshaft model (which is mostly identical to the 904) the parking lock is in the rear and locks the rear shaft.
For oil to come out of the dipstick hole, it seems logical that there has to be some kind of movement in the tranny internals and/or heat.
If the parking brake lock (inside the tranny) is engaging there shouldn't be any movement inside the tranny.
Put it in park and get behind your XJ and push (with the parking brake released ).
I've seen the parking pin (sprag) get nicked or dinged and hang.
Even with the transfer in neutral, the transfer internals can spin up some just from whatever little friction there is in the fluids.
If by any chance they moved the park sprag to the front in the 904, things can spin up inside parts of the tranny just from the fluid resistance inside the transfer and tranny. This will generate heat (there is no real circulation unless the motor is running). Thats why they recommend removing the drive shafts if you plan on towing at over 30 MPH with the tranny in neutral.
Get a mirror and trace down the tranny vent, find some way to blow some air into it and make sure it isn't clogged.

8Mud said:

If the parking brake lock (inside the tranny) is engaging there shouldn't be any movement inside the tranny. What about the driveshaft- will it be turning or stationary?
Put it in park and get behind your XJ and push (with the parking brake released ).
What am I expecting to see or not see?

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Thanks.

If the parking sprag is working properly, the transfer in gear and the tranny in park, with the drive shafts connected. The Jeep shouldn't move when pushed (or parked on a hill). I've seen the sprag not engage the shaft before, even with the tranny in park, you just think it's in park. Hope this makes sense. I've seen the srags stuck in and stuck out and even make partial contact that sounded kind of like a ratchet when the tranny was turning. This may not be your problem, but a possibility.
My thinking is, if the 904 has the same rear parking lock as the 727, there isn't really any way to build up any pressure in the tranny when flat towing. There shouldn't be any internal movement in the tranny. If the 904 has the parking lock in the front, there can be some internal movement in the tranny, probably the same as towing in neutral, which has a 30 MPH top speed recommendation, without removing the drive shafts. I don't have a blowup of the 904 that I've been able to find (yet), to check and see just where the parking sprag is in the 904 (I'm assuming it's in the rear, I may be wrong).
Clear as Mud .

jporvaz, unless your rear axle is broken, when the wheels are turning of course the rear driveshaft will turn. There is no way to disconnect the rear axle gears from each other. If the transmission is in park and mechanically locked from rotating, and the rear driveshaft is turning, then the only way to allow a disconnect is putting the transfer case in neutral. All autos that I've seen have the parking pawl in the back to hold on to the rear planetary ring gear as that is what drives the output shaft and transfer case.

Another problem you may run into while flat towing is that most t-cases while supposedly in neutral will lock the front and rear output shafts together, to the same speed. So if you're towing with both driveshafts installed down a twisty road (turns= different front and rear driveshaft speed) it's possible to induce enough bind to literally explode the transfer case. It's easy to check and see if the NP229 locks F/R together in neutral, if it does I'd just yank both shafts before flat towing.

When parked in my sloped driveway with tcase in gear, the jeep will roll 2 inches then stop when shifted from R to P so I would say the sprag is engaged.

I towed to Moab, UT (10 hrs driving time) in April and tow 1-2 hours around home almost every weekend without any problems until just the past 2 weeks.

I'm gonna check the vent and also fluid levels in the tcase and tranny.

Thanks for the suggetions- I'll crawl under the heep when it's a little cooler.

It's not unheard of for the tranny to push some fluid through the seals (the adapter) into the transfer. Unlikely for the transfer to build enough pressure to push fluid into the tranny and out the dipstick hole, without it running out someplace else, like the transfer vent, the rear seal or the front drive shaft seal. But heck most anything is possible, just sometimes unlikely.
If the transfer was pushing oil into the tranny, it would likely be low and the tranny overfull.
I've seen the tranny cooler inside the radiator fail and found coolant inside an auto tranny before.
If the transfer fluid has some water in it, it may boil the water. Steam (mixed with oil) is likely to blow out most anyplace.
I still can't picture the tranny (alone) building enough pressure to vent out the dipstick hole, with the rear shaft locked (park lock).

JJacobs is right, front or rear the parking lock should lock the output shaft. I was getting tunnel vision trying to picture a half dozen different kinds of transmissions where I'd seen the sprag or locking pin get hung up. Most of the malfs were in transmissions with the locking pin in the front.
Is it possible the dipstick is bouncing out of the tube and the overflow you are seeing is being sprayed out of the transfer vent tube into the area of the tranny dipstick? This seems like the most logical explanation.

At this point almost anything is possible. I have not done any extensive troubleshooting or fluid checks yet. Just got home from the drive yesterday and had it leaking on the way up and the way home. I'll pull it in the garage today and start crawling under it tomorrow. I have a new o-ring for the dipstick tube that has not been put in yet. I'll look for the vents and check the fluid levels. Something is going to be low with the amount of fluid that has escaped. As for the tranny cooler, I just put a new radiator in on Fri afternoon. I've had the leak both before and after the new radiator. I'll let you know what I find once I get underneath and start cleaning up the fluid.

jporvaz said:

At this point almost anything is possible. I have not done any extensive troubleshooting or fluid checks yet. Just got home from the drive yesterday and had it leaking on the way up and the way home. I'll pull it in the garage today and start crawling under it tomorrow. I have a new o-ring for the dipstick tube that has not been put in yet. I'll look for the vents and check the fluid levels. Something is going to be low with the amount of fluid that has escaped. As for the tranny cooler, I just put a new radiator in on Fri afternoon. I've had the leak both before and after the new radiator. I'll let you know what I find once I get underneath and start cleaning up the fluid.

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Please do, brain benders give me sleepless nights.

8Mud- your PM inbox is full.......

Well, here's the scoop so far. The tcase was almost a quart low on fluid. The tranny was 1 pint low. The tranny was full before I went on my last trip. I don't know about the tcase- I didn't check it:nono: .
The vent tube and hose are clear. I was surprised the vent hose runs to the back of the motor near the base of the distributor. But I blew through it and air flowed smoothly.

My thoughts at this point- the tcase was low on fluid and got hot, causing the fluid to flow into the tranny which became overfilled with hot tranny fluid and leaked out the fill tube- or- fluid got hot and vented through the vent hose and spilled down over the tranny.

Next step is to drive and see if there's leaks- and tow to see if it leaks.

jporvaz said:

My thoughts at this point- the tcase was low on fluid and got hot, causing the fluid to flow into the tranny which became overfilled with hot tranny fluid and leaked out the fill tube- or- fluid got hot and vented through the vent hose and spilled down over the tranny.

Click to expand...

Nope. Those are two separate systems, with seals and all that good stuff between 'em.

How are you checking fluid levels, by the way? The engine and tranny should be hot, and should have run through the selection of gears all the way from 1-to-Overdrive and Reverse. Stop the vehicle in a safe place on flat ground, and leave it in Neutral. (Use the e-brake or wheel chocks if needed) On flat ground in Neutral and with the engine running, check the fluid level in the tranny. THAT will tell you if it's full or overfull.

Overfilled, hot transmissions puke under load when hot.

Jim www.yuccaman.com

Tcase was checked cold, added fluid, then veh was started and run till tranny warmed up. Ran it through gears then checked with it N. Will need to get it on the road to do a more thorough fluid check and leak test.

If it turns out the fluid is spewing out of the transfer vent. A high performance Dexron can reduce foaming and the spew. Another fix is to find a clear fuel filter, split it, take out the filter part, glue the two halves back together again and clamp it and the vent cap with a short piece of tubing on top of the filter and back on the vent line. This allows the foam/fluid room to expand some before it gets to the end of the vent line/cap. The foam turns back into a liquid and will drain back down the vent line. Not a hundred percent fix, but can reduce the spew by 90%.
You can usually tell if Dexron has been overheated, by the color and the smell. Cherry red is good, red to straw colored is OK, dark straw to gunk brown is bad and will often smell burnt. It will also get gummy/sticky and thick.
Like Yucca-Man said the transfer and tranny are two separate pieces with an adapter in the middle. I've seen one or the other leak fluid into the adapter, but have never seen them swap fluid, though one guy did say it happened to him.